The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines combust a fuel and air mixture to produce drive torque. More specifically, air is drawn into the engine through a throttle, which meters the air flow. The air is mixed with fuel and the air and fuel mixture is compressed within a cylinder using a piston. The air and fuel mixture is combusted within the cylinder to reciprocally drive the piston within the cylinder, which in turn drives a crankshaft of the engine. Combustion is initiated by creation of a spark in the cylinder by a spark plug. Spark timing is provided in terms of the angular position of the crankshaft relative to the piston achieving a particular position (e.g., top dead center (TDC)) within the cylinder, at which the spark is initiated.
There exists an optimal spark timing which produces the maximum engine torque for a given engine speed and mass of air and fuel mixture within the cylinder. This is called Maximum for Best Torque, or MBT. Combustion of the air and fuel mixture within the cylinder has variability with respect to the amount of torque created for a given spark advance and for a mass of air and fuel mixture. Combustion variability has many sources and will not be discussed here, but it causes a variable engine torque, which in turn causes variable engine speed.
During idle, it is desired to operate the engine at a predetermined engine speed. Since combustion variability causes varying engine speed, an engine idle speed control system should be able to compensate for this.
One method is to retard the average spark advance away from spark timing for MBT, and compensate engine speed error by changing spark advance on an individual cylinder firing event basis. Because this average spark timing is sub-optimal for a given fuel/air rate, the engine produces less torque than at the optimal spark timing. If additional torque is needed to increase the engine speed, the spark timing is advanced closer to spark timing for MBT to produce additional torque for same air/fuel rate. Similarly, if less torque is needed because the engine speed is higher than desired, the spark timing is retarded. The spark timing is adjusted because it can be executed much more quickly than changing air/fuel rate.
By regulating spark timing to less than optimal, this method of engine idle speed control maintains a torque reserve, which is the difference between the torque produced by the engine with spark timing for MBT, and the torque produced retarded spark timing. The torque reserve is established at a level that will maintain the engine speed above a predetermined minimum speed in the event of a large, unanticipated torque load on the engine (e.g., a full cramp of the power steering). Engine idle speed control generally utilizes only a maximum of approximately 30% of the torque reserve to account for combustion variability.
The amount of torque produced by a given change in spark timing for a fixed air/fuel rate depends on the current spark advance, the current air/fuel rate and the current engine speed. The relationship between engine torque and spark timing at a given air rate is described as non-linear curve that varies as the air rate changes and that flattens out as spark timing is advanced closer to MBT. This can present a problem to the idle speed control system.
If the torque reserve value is large, the change in engine torque for a change in spark timing (i.e., slope of the curve) over the normal spark control operating range is relatively constant. Therefore, a change in spark timing as a function of the difference between desired and actual engine speed maintains the engine sufficiently well with an air flow and spark timing invariant gain. The average spark timing is roughly 13° before TDC (BTDC) for an exemplary engine, and stays between 10° and 16° BTDC for an idle condition where loads are not changing, but combustion has typical variability. If the torque reserve is reduced, as may happen for a system with smaller unanticipated loads, the actual range of operation for spark timing changes and the slope of the curve can change rapidly over the normal spark control range.
Both the shape of the curve and the slopes vary widely between the curves for each air flow rate. As a result, using a spark timing invariant gain does not deliver acceptable engine speed control. This problem becomes compounded with differing engine idle speeds, which happens as the engine warms up. As a result, the required spark timing gain must be scheduled from a table, with multiple tables required to cover the range of engine idle speeds encountered during engine operation. It is very time consuming and cost intensive to accurately populate such a set of tables.